1. Field of the Invention
This invention relates to explosive gas detectors of the catalytic type, and particularly to such a detector wherein means are provided to limit the temperature rise of the catalytic element employed.
2. General Description of the Prior Art
It is known that explosive gas can be detected by an electrically heated wire having on it a catalytic material. Explosive gas causes a reaction with the catalytic material which produces additional heat, and this in turn changes the resistance of the wire. By metering the changes in resistance, as by the employment of an electrical bridge circuit, an indication of the concentration of gas is obtained.
There are several difficulties with this type of detector. A principal one is that the operation of the device depends on elevated temperatures of a catalytic element to provide indications, and the elevated temperatures in turn shorten the life of the element. A second difficulty is that with use, the catalytic element changes character, and this requires recalibration of the detector. A third difficulty is that at high levels of gas concentration, there is a tendency for the device to provide a lower than actual indication of gas, this occurring because of a thermal equilibrium which sometimes arises by virtue of a balance between reaction caused heating of the element and radiation given off by the element. A fourth difficulty is that when operated at higher temperatures, the catalytic element has a tendency to fail in the presence of significant vibrations, and this is particularly a problem when a detector is used on or near mining machinery, as in a coal mine.
It is the object of this invention to overcome the above and other difficulties and to provide an improved explosive gas detector which is both accurate and has a long operating life.